There are like a million games on sale so ITT link good deals and hidden gems everyone should look into.

You BETTER have these:
Trine $8 - Platformer with some rpg elements, really awesome style
Torchlight $10 - Action RPG, tons of loot, neat visual style, loot.
Aquaria $10 - Awesome 2D Metroidvania-like. Beautiful graphics and amazing soundtrack.
The Witcher $14 - RPG, really good story (it starts slow though)
Borderlands $33 - FPSRPG, this is like the 5th time it has been on sale, if you don't have it yet then you're pretty much a bad person.
Penumbra Pack $10 - FPS/Adventure, one of the most frightening games I have ever played.

Look into these neat games:
Machinarium $10 - A charming little adventure game where you're a robot in robotland.
Mirror's Edge $5 - FPS, you run places.
STALKER $2 - FPSRPG, real immersive, kinda scary. (Also, the sequel Clear Sky is $5)
World of Goo $5 - Clever physics based game where you construct things out of blobs to pass obstacles.
Armed and Dangerous $5 - A third person platformer/shooter from the team that made Giants: Citizen Kabuto. I remember this being funny.
Plants vs. Zombies $7.49 - Really, really addictive defense game.
Zeno Clash $7.49 - FPS, delightfully bizare punching adventure using the Source engine
Dark Messiah of M&M $5 - FPSRPG, if you haven't played this I will kick you into a wall of spikes.

Also maybe worth your time:
Twin Sector $10 - FPS Puzzle/Platformer, demo was neat.
Samorost2 $2.49 - Adventure/Puzzle game. Short but charming.
LucasArts Adventure Pack $2.49 - 4 classic adventure games.
Windowsill $3 - Not actually on sale, but it is $3 and a really neat though short puzzle. Great animation.
Shadowgrounds Pack $4 - Top-down shooters that pit you against hordes.
Osmos $5 - A kinda calm game built around physics and gravity.
The Void $10 - Weird experimental game, has good reviews though I haven't played it yet myself.
Cold Fear $10 - Take RE4 and put it on a boat and you have Cold Fear. This is not a bad thing.

Also, they have the Rainbow Six Vegas games on sale for 50% off each ($10 for R6V and $5 for R6V2, yes that doesn't make sense)
I have both of these on the 360 and they're good games but they have awesome coop. If anyone else wants to play these coop we could get it together.

Space Quest Collection $5 - Six wicked hilarious adventure games. They're not for everyone, but if you are the kind of person that these games are made for then there's a good chance you'll love the games. Here's a youtube video of some guy going through one of the zones in Space Quest IV smelling and tasting everything in sight: http://www.youtube.com/watch?v=4nCjb9oinNU

The games run on DOSBox and apparently there's some sort of front-end so you don't have to configure DOSBox.

I would buy this collection but I have two copies of all of these games already.

Crayon Physics $10 - A really cool physics based game where you have to figure out how to get the ball to the goal, and you do so by drawing things to launch, pull, or push the ball. You build things by drawing them on the screen, like catapults, seesaws, pulleys, or really anything you can imagine. It's really addictive. The levels get really insanely hard to figure out, so if you like a good challenge then this is a good game for YOU.

One thing that really bugged me with Crayon Physics is that it didn't have a pause / go / restart feature. If you made some elaborate thing and it fell into the abyss you were back to square one and had to draw it all over again. Maybe the fixed that.

Civ 4 Complete $27 - It's nowhere near the cheapest game on sale but it's so good that that doesn't really matter, it's still a really good deal. There's also Beyond the Sword for $13 if you have vanilla Civ 4 already but haven't gotten any of the expansions (this is the only one you need); despite what the store page says, you don't actually need the Steam version of the base game, it'll work with retail (and probably other digital distro places') versions.

Telltale Everything Pack $50 - I haven't played all of these but I can confirm that Tales of Monkey Island ($25 on its own) is really good if you want some adventure games that aren't ridiculously challenging or unforgiving but are still fun to play through.

Defense Grid is only gonna be $2.50 for another hour or so but it's so good that it'd be worth the full price. Even if you don't normally like TD games, Defense Grid is just so well presented and executed that you'd most likely really enjoy it anyway.

Also it's worth pointing out that Fallout 3 GOTY Edition is on Steam now and it's $38; considering that the DLC alone is $50 altogether it's a really good deal if you haven't gotten any of it yet.

Here's hoping that Dragon Age is one of the daily deals, I'm regretting not picking it up when it was on sale last month. :I Also I played a little bit of The Void and if weird, pretentious-sounding stuff is your thing then you'll really like it (I really like it).

Buy Torchlight. It's a single player Diablo 2 clone with stylized graphics and it's done very well.

Buy the Max Payne bundle. Third person shooters where the gimmick is slowing down time. MP2 has some great storytelling, too. One of my favorite games ever.

Buy Dawn of War 2. It is a semi RTS semi action RPG where you control 4 space marine squads at a time and go around killing bad guy aliens and completing missions. It's pretty RPG-ish in the sense that you level up, have stats, get abilities, have equipment, find items on the battlefield, but RTSish in the sense that you need to use cover, match certain units against other units, think strategically, use your resources wisely, etc. Haven't tried the multiplayer but I hear it is more RTS-ish like the first DoW

Buy the Lucasarts Adventure Pack. Four really good adventure games for $3.

Buy Audiosurf. Music based puzzle game that uses your MP3s to create 'tracks' that you use your 'spaceship' to fly over and collect different colored blocks to match 3+ at a time. Or you can ignore that like me and just do the mode where you avoid all grey bricks and catch all colored ones. It's really hard.